As a network storage system employing the NFS (Network File System)/CIFS (Common Internet File System), as a protocol, and which enables accessing the file systems from plural hosts, the NAS (Network Attached System), for example, is currently used. The NAS means a computer or a device co-owning files via NFS and CIFS.
Data recorded in a storage are periodically backed up on a medium, such as a tape, in order to make it possible to restore data for the possible case e.g. of occurrence of malfunctions. Of these backed-up data, a snapshot is a read-only file system holding an image of a file system structure at a certain point in time. By taking the snapshot, a fixed image of the file system structure at the time point of formation of the snapshot may be accessed, while data at the time point of formation of the snapshot may be restored, that is, the logical volume may be restored. For this reason, the snapshot may be used as an example for taking consolidated backup of the snapped file system. As for the snapshot, reference may be had to e.g. the Patent Document 1 and the Non-Patent Document 1. According to the present invention, the processing for interlinking the plural snapshots is carried out, as later explained. For aiding in the understanding of the relevant technology, the snapshot will now be explained, based on the description of the Patent Document 1. Meanwhile, in this Patent Document 1, there is disclosed a method for managing a plurality of snapshots in case these snapshots are supplied. With this method, a series of metadata representing a file system are included in the file system, so that snapshot of the file system will include relevant metadata.
If an image of a file system structure at a certain point in time has snapped a file system as shown for example in FIG. 13A, a snapshot inode, shown for example in FIG. 13B, is generated as an image of a file system structure at a certain point in time. Meanwhile, the inode is a data structure for a file system layer to specify a file. A snapshot inode is a replication of a root inode of a file system structure and includes the pointer information to an inode indirect block, an inode file data block and to file data blocks A to C, as in FIG. 13A. Since the snapshot may be generated by simply acquiring the pointer information, the time needed for generating the snapshot is on the order of several seconds, even for a large volume of data. If, after generating the snapshot, the file data block C, for example, is changed, the inode file data block, pointing to the so changed file data block, is changed to reflect a new location C′ of the so changed file data block (in the case of WAFL (Write Anywhere File Layout)), as shown in FIG. 13C. The snapshot inode includes a pointer pointing to the original inode file system indirect block, while the inode file data block includes pointers to the original file data blocks A to C. That is, the snapshot holds a fixed image of the file system as of the snapping time point. On the other hand, the newly written inode file data block holds pointers to the file data blocks A and B and the pointer information to the changed file data block C′. There is also known a configuration in which, when a snapped file system is subsequently changed, pre-change data, such as the file data block C of FIG. 13C, is copied to a snapshot area, and in which, in reading the snapshot, the changed original data, such as the file data block C of FIG. 13C, for example, is read from the snapshot (see e.g. Non-Patent Document 1).
If, after generation of a snapshot, a file is changed, the inode structure of the file system may be restored to that at the time point of snapshot generation, by accessing the snapshot inode. More specifically, by following the pointers retained by the snapshot inode, through the inode file indirect block and the inode file data block, up to the unchanged file data blocks A to C, the file system structure may be restored up to the state at the time point of formation of the snapshot. It is noted that the ‘SNAPSHOT’ is a trademark owned by Network Appliance Inc. However, of course, the ‘snapshot’ in the context of the present specification is not limited to the snapshot directed to the WAFL (Write Anywhere File layout) of Network Appliance Inc. and means a fixed image of a file system.
As for use of the snapshots, the date and time of snapshot generation may be recorded, by way of generation management, and plural snapshots may be managed within the same file system, depending on the particular type of the snapshot forming software. The file system structure snapped may be accessed via e.g. an alias.
The basic configuration of the backup/recovery of the file servers in the conventional network file system environment is the individual backup/recovery by each file server. In this case, a client accesses a file server, such as NAS, by e.g. an NFS protocol, over a network, to make a remote login. Or, the client makes a local login to take a backup on e.g. a local tape drive, using CLI (Command Line Interface), such as rsh or ssh. Alternatively, such a method may be used in which a backup server, as a computer responsible for taking a backup, is NFS/CIFS mounted to a file system of a backup client (file server computer), to take a backup. Still alternatively, a backup client (file server) may be connected to a backup server, using a protocol other than the NFS protocol, such as SCSI (Small Computer System Interface), as with the NDMP (Network Data Management Protocol) prescribing the backup/recovery communication protocol between a primary storage (storage for storing a file system currently in use) and a secondary storage (storage for data retention), or a tape library may be co-owned by plural file servers.
There is also known a configuration in which a switch 100′, virtually consolidating the resources of plural file servers 3, is logically provided across the file servers 3 and the client 1, for consolidating the plural file systems, in order to provide file services of a virtually single system image, that is, SSI (Single System Image), as shown in FIG. 14 (see e.g. Patent Documents 2 and 3, indicated hereinbelow). This switch 100′ provides file services of the single system image, such as a single NAS, by consolidating and managing plural servers, without tampering with pre-existing systems, provided only that the client and the server support the standard protocol (NFS). Reference is also made to the disclosure of, for example, the Patent Document 4, as a similar virtual file system technique, a system for virtually unifying plural network storages to a single storage, to enable accessing from the client to, as it were, a single network storage. The system differs in configuration from the switch 100′ of FIG. 14.    [Patent Document 1]
JP Patent Kokai Publication No. JP-P2004-38929A    [Patent Document 2]
JP Patent Kokai Publication No. JP-P2003-203029A    [Patent Document 3]
JP Patent Kokai Publication No. JP-P2004-54607A    [Patent Document 4]
JP Patent Kokai Publication No. JP-P2003-345643A    [Non- Patent Document 1]
‘HP JFS 3.3/HP OnlineJFS 3.3VERITAS@ File System 3.3System Management Guide’, 4Online Backup, p75to 81